After four years in the emergency room, Wen had seen her share of accidents, shootings, even amputations. But she says nothing in her eight years of medical training prepared her for that day.

"I am glad I was able to help," says Wen, 30, a physician in her last year of a four-year residency at Massachusetts General Hospital. "I wish I could have helped more. But I wish I hadn't seen it."

Wen was nearing the end of her shift when a call came over the loud speaker, announcing that the hospital was about to get two patients injured in an explosion. As a top-level trauma center, treating two patients would be manageable, Wen thought. With some doctors arriving just as others were about to leave, the hospital had twice the usual number of physicians.

Minutes later, another call came in, explaining that there had been two explosions.

The ER was already completely full, Wen said. Dozens of other critically ill patients, including ones on respirators, had to be relocated to other parts of the hospital. Wen and other staff got everyone cleared in 30 minutes.

All too quickly, the ER was full again, with patients in more distress than Wen had ever seen. Some were silent; some asked repeatedly about family members. Others cried and screamed.

"At some point, I wanted to cry with them, but I couldn't," Wen said. "There was blood everywhere. On the floor, there were trails of it behind the stretchers."

There was also soot from the explosions, and the smell of burned clothing, and burned flesh.

One of her most important jobs was triage: deciding which patients needed immediate treatment, which should be first in line for the operating room.

With so much blood everywhere, it wasn't always easy to identify just where the bleeding was coming from. Once she located the source of bleeding, she worked to stop it, tying tourniquets on those in danger of bleeding to death. She performed CPR on patients without a pulse. She ordered blood and fluids to replace what patients had lost.

Wen had been working since 7 a.m. without a break before the bombing, and she didn't get a break after. "I wasn't weak," she said, "because my adrenaline was so high."

Even as she rushed to save lives, Wen's attention was divided. She kept an eye on the scene around her, to make sure that there weren't other patients who needed her more.

A phone in the ER beeps an alarm with every new patient. On Patriot's Day, those alarms never seemed to stop.

Thirty-one of the more than 260 people injured that day were sent to Massachusetts General. Most seemed to arrive within the first 90 minutes, Wen said. At least four required amputation.

One of the most severely injured was Marc Fucarile. His left leg was fractured in several places. His right leg needed to be amputated. Burns covered half his body. Shrapnel was embedded everywhere, including his heart, said his fiancée, Jen Regan, on Monday.

Wen couldn't help worrying about her own husband, who had gone to the finish line to see friends. Wen and her husband live just a block away from the site of the explosion at Copley Square.

"I was terrified that the next person I would be resuscitating would be my husband," said Wen, who didn't learn he was OK until two hours later.

"We kept hearing reports that there had been other explosions. We had no idea what was happening around us. We had no idea whether we would walk outside and see a war zone."

The staff at Massachusetts General had been through emergency drills, and everyone knew what to do, Wen says. "We had people coming from all over the hospital asking, 'How can I help?' "

But no one felt prepared for that day.

"I've never seen a shrapnel injury," Wen said. "That's just not something we see in an urban city in the U.S.

"It was very difficult to see so many people's lives changed in one day. Seeing all those young people who were my age, who would have very different lives moving forward, if they had lives at all."

But even after a 12-hour shift, she had no place to go home to. Because her apartment was so close to the crime scene, police had the area blocked off until late in the night.

More than a week later, Wen still has nightmares. Every new alarm beep takes her back to the day of the bombing, when it seemed like the number of patients coming through the doors would never end. Her neighborhood and all the places she usually goes - her bank, her grocery store - remain shuttered.